When press forming a sheet blank into a metal part having a shape bent along the longitudinal direction and a box or hat section, after removing the metal part from a forming die, the bending stress from the die is released and the formed metal part returns toward the releasing direction of the bending stress (springback occurs). The springback often occurs in the bent portion of the metal part, and the dimensions of the bent portion of the formed metal part cannot be controlled with high accuracy.
The conventional forming die considers the predicted springback, and the shape of the die is modified so that the desired dimensions of the formed part are obtained. In recent years, the metal part is required to be light and of high strength, so that the metal part is often made of high-strength steel. The high-strength steel leads a large amount of springback after press forming, and it is difficult to keep accurate dimensions, therefore the forming die may be adjusted time and again. An increase in number of adjusting may lead an increase of production time and costs.
A technique of solving the above-mentioned problems is disclosed in JP 2007-222906 A or JP 3864899 B1.
JP 2007-222906 A discloses the press forming method for forming a metal part having a curved shape in the longitudinal direction, which includes a first forming step of forming an intermediate product having protrusions at the top surface and a second forming step of pressing the protrusions to make the top surface flat and forming the final product. According to the technique, the tensile stress in the longitudinal direction may be reduced and the amount of springback may be decreased.
The metal part formed by the forming method according to JP 2007-222906 A has the flat top surface. As shown in FIG. 14(a), the protrusion 120, which is formed on the top surface 110 of the metal part 100 in the first forming step, is pressed in the second forming step, and the pressing process occurs a stress differential between the front surface and the back surface in the direction perpendicular to the longitudinal direction (width direction) of the metal part 100. Thereby, the springback may occur in the width direction, which also leads the springback in the longitudinal direction of the metal part 100 as shown in FIG. 14(b), and thus it is difficult to keep the dimensional accuracy.
JP 3864899 B2 discloses the press forming method including a preliminary forming step of forming the top surface and the walls by using the small punch and a finishing step using the finishing punch. The small punch bends the walls to the direction opposite to those of the final metal part, thereby reducing the springback of the walls after finishing.
During the finishing step, the finishing tool is used, which includes a punch for forming the predetermined section in which the top surface has the recess, and a dice having the projection facing the recess of the punch. The top surface is formed with the projection projecting inwardly just before the lower dead end of the finishing stroke.
As shown in FIG. 15, in the press forming method of JP 3864899 B2, during the finishing step using the finishing punch 160 after the preliminary forming using the small punch 150, the wall 170 is not entirely held by the die set 180. Therefore, there may occur wrinkles or cracks in the formed metal part. Particularly, as shown in FIG. 15, at the boundary portion 175 between the wall 170 and the top surface 190, the wrinkles or cracks may easily occur.